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Q: I was taught at school that the three primary colours are red, blue
and yellow and that by mixing these, all the other colours could be made.
If you look closely at a television screen, you can see that the picture
is made up of red, blue and green dots. Why and how do televisions use
these colours and how, for instance, do they make yellow from them?

* * *

A: There are actually two sets of primary colours. The red, blue and
green set is known as the additive set and is the one relevant to mixing
light. The other set, consisting of red, blue and yellow (technically magenta,
cyan and yellow, as it is impossible to produce magenta or cyan from red,
blue and yellow), is known as the subtractive set and is considered when
mixing paints.

The reason for having two sets is that the processes involved in the
coloration of the media are different. With the subtractive set, chemicals
in the paint absorb light of some wavelengths more than others, and the
colour consists of what light is reflected by mixes of chemicals. The additive
set, however, shows the colour of the light, and when you combine two of
the primary colours, your eye interprets the frequencies as a secondary
colour.

So television screens use red, blue and green because they mix lights,
not paints.

Edward Catmur Esher, Surrey

* * *

A: Televisions start with a black screen and add coloured light. Painters
start with white paper and add paint which serves to remove colours from
the reflected light. The television screen uses additive colours, so called
because adding more makes the image brighter. Painting uses subtractive
colours and works in the opposite way round.

As you might expect, the two systems complement each other. Adding yellow
and blue paints together gives green, which is an additive primary colour.
Conversely, yellow light can be made out of red and green light.

Here’s where the fun starts. The subtractive primaries are technically
yellow, cyan (an intense light blue similar to sky blue) and magenta (a
kind of purple). True red paint can be made from yellow and magenta paint.
The problem is that it is difficult to get a really good magenta, and so
artists end up using red instead of magenta. Although this precludes being
able to mix a good, clean magenta, it is rare to find one in the environment
anyway, so it’s not missed. However, without magenta, it is difficult to
make a good, deep blue from cyan and so ultramarine is used instead. The
effect of this is that green is now difficult to create, and so bright green
is used. So really, although artists claim to use yellow, blue and red,
they in fact use a couple of reds, yellow, green, blue, a large tube of
white, black, and often an assortment of tertiaries. Painting is an engineered
solution.

Gavin Peacock Brighton, East Sussex